1. Field of the Invention
The present invention relates to a method for estimating and compensating transceiver non-idealities. The non-idealities may be transmitter and receiver IQ imbalance and/or carrier frequency offset.
2. Description of the Related Technology
The effect of IQ imbalance in a transmitter or receiver is that it produces an unwanted distorted signal that is superimposed on the original desired signal. In order to reduce or even eliminate this unwanted distorted signal one needs to estimate the ratio between the original signal and the distorted signal.
The preamble preceding the data in a communications burst, usually contains known training sequences; Golay sequences can for example be used. When there is IQ imbalance in the communication system, the preamble training sequences are modified by the addition of the unwanted distorted version of the signal produced by the IQ imbalance. This impacts the ability of the receiver to make the correct estimates of carrier frequency offset (CFO) and channel response and finally recover the transmitted data.
In the prior art, several methods are proposed for performing joint CFO and IQ estimations. If the first part of the preamble contains more than three repetitions of the same sequence, following methods can be used for the example with Golay sequences: “Feng Yan et al., Carrier Frequency Offset Estimation for OFDM Systems with I/Q Imbalance, Proc. the 47th IEEE Int. Midwest Symposium On Circuits and Systems, MWSCAS '04, Vol. 2, pp. 633-636, July 2004” and “De Rore S. et al., Joint estimation of carrier frequency offset and IQ imbalance for 4G mobile wireless systems, Communications, 2006 IEEE International Conference on, vol. 5, pp. 2066-2071, June 2006”. The result of these two methods is an estimate of cos φ where φ is the angle rotation caused by the CFO over a known time interval. This angle rotation is directly proportional with the CFO. Using the acquired CFO estimate, it is then also possible to make an estimate of the IQ imbalance ratio. The methods fail when the CFO is small. For small CFOs, the cos φ estimate is too imprecise for an accurate CFO estimation and also subsequent estimation of the IQ imbalance becomes inaccurate and useless.
In “De Rore S. et al., Joint estimation of carrier frequency offset and IQ imbalance for 4G mobile wireless systems, Communications, 2006 IEEE International Conference on, vol. 5, pp. 2066-2071, June 2006”, a method is proposed that works for small CFOs but needs a specially designed preamble sequence that is not available in the 802.15.3c case. For OFDM systems several methods have been proposed that jointly compensate CFO and IQ imbalance; some are applicable to OFDM systems in general like “J. Tubbax, B. Come, L. Van der Perre, S. Donnay, M. Engels, M. Moonen, and H. De Man, “Joint compensation of IQ imbalance and frequency offset in OFDM systems,” Proc. of the IEEE Radio and Wireless Conference, pp. 39-42, Augustus 2003” and “S. Fouladifard and H. Shafiee, “Frequency offset estimation in OFDM systems in the presence of IQ imbalance,” Proc. International Conference Communications, vol. 3, pp. 4280-4285, May 2003”, others use special pilot training sequences “Guanbin Xing, Manyuan Shen, and Hui Liu, “Frequency offset and I/Q imbalance compensation for direct-conversion receivers,” IEEE Transactions on Wireless Communications, vol. 4, pp. 673-680, March 2005.” In the 802.15.3c case, the special training sequences are not available and besides OFDM, the standard also defines single carrier systems, intended for use with- and without frequency domain equalization.